The present invention relates to a method of controlling operation of a winder for a fiber web.
On a slitter-winder of a fibrous web, such as paper or board, a full-width web is unwound from a so-called machine reel and the web is slit into several partial webs and the partial webs are wound into so-called customer rolls.
The operating process of the slitter-winder mainly comprises a so-called set change and a slitting process as successive steps. The slitting process may be considered including an acceleration step after the set change, a normal slitting step and a deceleration step preceding the set change. Of these, the normal state slitting step takes the most time by far. The web speed of the slitter-winder can typically be even 50 m/s. The roll formation in a winder is controlled by effecting on various variables depending on e.g. a type of a winder.
As an example, a commonly used type of a winder is a so called carrier roll winder, in which the set of rolls is supported by two carrier elements, such as two king rolls or a drum and a belt assembly. In such a winder the roll formation is mainly effected by the used winding forces between the roll and the support drums, web properties, as well as the nip load.
The partial webs are brought to the winder via a drum, which in a carrier drum winder is the rear drum and in a center wind winder the center drum. In a carrier drum winder there is a front drum (or a support belt assembly) provided, which together with the rear drum forms the winding cradle on which the set of wound rolls are being wound. Additionally, the partial web rolls are supported by a press device, typically a press roll, generally above the set of rolls. In the carrier drum winder both the front and the rear drums are driven, typically by a dedicated motor. In a centerwind winder the center drum is driven, but also the web roll itself is driven at its winding core or a shaft.
Recently, the efficiency of the slitter-winder has been improved considerably by increasing running speeds, among others. The total efficiency is naturally affected by efficiency in all above steps, and therefore used speeds and accelerations are typically intended to be maximized. Therefore, it is evident that reeling forces are also maximized, but within the limits of properties of the winder and the web.
It is common practice in connection within a field of control of electric motor drives with frequency converter to apply torque control. A set torque reference range is followed by the drive. If the range is for any reason exceeded the speed controller activates and takes over the control. It has been discovered that this kind of procedure does not provide a solution fast enough for adequately preventing loss of traction in an application of the web winder.
U.S. Pat. No. 6,089,496 A discloses a method of controlling operation of a winder for a fiber web in which method fiber web is brought on the web roll via a nip formed by a first support drum and the web roll which first support drum is driven by a first drive assembly applying controllable torque to the drum, and applying winding force to the web roll by a second drive assembly. According to the document the torque and nip loads for changing the tension remaining in the wound web roll operate such that the tension of the web first decreases at increasing web roll diameter of said at least one web roll during an initial winding phase, then stays approximately at the same level and, after winding further, decreases further at increasing wound web roll diameter during a final winding phase.
GB 2117935 A discloses a method of controlling the internal tension of a web roll, e.g. a paper web roll, during winding of the roll in a winder having two individually driven supporting rollers, the rotational speeds of the supporting rollers or their drive members are measured and the speed signals are fed to control means to maintain a desired speed difference therebetween.
EP 2133298 A2 discloses a method of optimizing the operation of a device to roll up a sheet of material in a winder. The winder comprises carrier rolls the drives of which are individually controlled such that the first support roller is speed controlled and the second support roller is torque controlled.
The existence of friction in various moving and rotating parts of a winder is known as such, an example of which is referred to US 2008197228 A1, which discloses a method for friction compensation in a winding machine, with which a material is wound onto a winding drum, and the winding drum is driven by a winding drive which is triggered by a control/regulating device, and in the control/regulating device a driving torque of the winding drive is specified, and in a friction compensation unit, as an input-side process parameter, a winding speed of the winding drum is taken into account, in which to compensate for the frictional torque, at least one additional process parameter is taken into account.
U.S. Pat. No. 3,910,521A discloses a winder control for programming the torque to be applied by a winder to effect winding of material into a roll wherein the tension applied to the material to be wound is dependent upon the instantaneous radius of the roll of wound material, the winder includes first sensing means for sensing the angular velocity of the roll of material and producing a signal indicative thereof, second sensing means for sensing the linear velocity of the material to be wound, and producing a signal indicative thereof, and divider means for generating a signal indicative of the instantaneous radius of the roll of wound material. The radius signal is directed to means for multiplying the instantaneous radius signal by a factor indicative of the desired tension to be applied to the material when the roll of material has a predetermined radius. This establishes a torque signal indicative of the torque the winder must apply to the roll of material to obtain the predetermined tension in the material to be wound at the instantaneous radius calculated by the divider means.
Even if the torque would be effectively controlled, when maximising the productivity of the winder, it is evident that the winding forces transmitted by the drives over the nips have a tendency to be at the limits of capability of the nip of transferring the force without hampering the quality of the web or without slipping of the counter surfaces in the nip. The capability of a nip to transfer force is mainly dependent on nip force and a friction coefficient in the nip.
An object of the invention is to provide a method of controlling the operation of a winder in which the performance is considerably improved compared to the prior art solutions.